Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms are categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ).
The first technology, “drop-on-demand” ink jet printing, provides ink drops that impact upon a recording surface by using a pressurization actuator (thermal, piezoelectric, etc.). One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TIJ).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink may be perturbed in a manner such that the liquid jet breaks up into drops of ink in a predictable manner.
Printing occurs through the selective deflecting and catching of undesired ink drops. Various approaches for selectively deflecting drops have been developed including the use of electrostatic deflection, air deflection and thermal deflection mechanisms.
There is a need in the CIJ industry to track the print media during the printing process. For example, US Patent Publication 2006/0132523 discloses a printer having a single 2D optical encoder having a coherent or quasi-coherent light source that reflects light from the print media that is received by a detector for tracking the motion of the print media.
Although satisfactory, this method includes shortcomings. One shortcoming is that the print media is monitored at only one location which limits the available information on the location of the print media. A second shortcoming, particularly in web-based systems, is that stretch of the print medium in the in-track and cross-track directions is not available. The present invention addresses and solves these shortcomings.